This invention relates to a stent for use in a body passageway, comprising a flexible self-expandable braided tubular wall. The invention also relates to methods for manufacturing such a stent.
Use of expandable stents is known for damaged areas of body vessels such as for example food pipes, for dilatation, repair or bridging such areas. Where a patient suffers, for example, from a cancer of the esophagus while being otherwise in good status, stenting is a valuable approach to help him live. As such stents are subjected to stresses, in particular due to movements of the duct such as peristaltic movements, there is a tendency for the stent to migrate along the duct where it is placed. When the stent is used for a tumor at the end of a food pipe, for example at the junction of the esophagus and stomach, the problem of migration is further enhanced because the stent may have to protrude into the stomach. In such a case, the problem of anchoring the stent in the duct becomes particularly critical because the stent may fall into the stomach. A further problem arising with stents is that they have a tendency to close the pipe in curved areas thereof because of their deformation.
The document xe2x80x9cEndoscopy 1992:24:416-420xe2x80x9d describes a covered expandable metallic stent for preventing ingrowth of malignant structures. This stent is made of a steel wire bent in a zig-zag pattern and the stent legs are connected to wire skirts at each end which are intended to improve anchorage of the stent in a body passageway. In addition, 1 mm. barbs are attached to the skirts to still further enhance anchorage of the stent in the passageway. However, the document specifically outlines that migration remains a problem despite the wire skirts and barbs which were provided for anchorage purposes. Such a structure will certainly not allow safe anchoring of the stent in a condition where the stent cannot anchor at one of its ends, as in the case of a tumor at the end of the esophagus. And there are no solutions to overcome the pipe closure due to deformation of the stent in curved areas.
U.S. Pat. No. 4,655,771 discloses a stent made of a flexible tubular braided structure formed of helically wound thread elements. When the stent is deployed the stent assumes a substantially cylindrical shape as it expands and substantially conforms to the vessel wall, and the document outlines that such an expansion allows the stent to stay in place by self-fixation because of the permanent pressure of engagement against the vessel wall. Such a configuration may provide a good fixation in smooth rectilinear areas of the vessel. However, it will not provide a safe fixation in areas where a part of the stent cannot bear against the vessel wall. Nor will it solve the problem of pipe closure in curved areas of the vessel.
U.S. Pat. No. 5,064,435 shows a body implantable stent consisting of two or more generally tubular, coaxial and slidably joined stent elements each of which is of open weave construction, formed of multiple braided, helically wound strands of resilient material. The stent is thus elastically deformed to a reduced radius when deployed and it self expands radially when released after positioning in a vessel or other body cavity. To match the axial contraction of the stent upon radial expansion thereof and preserve a consistent length of the stent in spite of the axial contraction of the overlapping stent elements, the axially outward and non-overlapping portions of the stent are designed as radially outward flares to secure fixation of the stent to the vessel wall. Accordingly, axial contraction of the stent occurs as a reduction in the length of the medial region where the stent elements overlap. Other means to maintain the axial length comprise reinforcing filaments near the opposite ends of the stent elements to increase the restoring force, or fixation of hooks at the opposite ends of the stent elements, or still an elongate axially directed flexible and inextensible wire secured to the opposite ends of the stent elements. Such a configuration cannot be safely used if both the ends of the stent elements are not very strongly affixed to the vessel wall. As a matter of fact, if one of the stent elements is not firmly secured to the vessel wall, it may migrate with respect to the other stent element, for example because of peristaltic movements, whereby there may be a separation of the overlapping stent elements; where the stent is to be used at a place such as the junction of the esophagus to the stomach, the unsecured stent element will fail into the stomach. Complete separation of the stent elements will not occur in the case of use of an inextensible wire secured to opposite ends of the stent elements; however, such a wire cannot prevent part separation of the stent elements, for instance where the stent takes a relatively sharply curved configuration, which may cause serious injury to the vessel wall. And furthermore, whatever its configuration, the overlapping arrangement may still enhance the problem of pipe closure in curved areas because of the reduced flexibility resulting from the overlapping condition of the braided structure.
It is the primary object of the invention to avoid the aforesaid drawbacks. A further object of the invention is to provide a stent structure which allows safe and efficient operation in critical areas such as the end of a food pipe. Still a further object of the invention is a stent which minimizes the risk of pipe closure whatever the configuration of the body passageway. And it is also an object of the invention to provide for methods for manufacturing such a stent which are simple, efficient and economical.
Accordingly, the flexible self-expandable braided tubular wall forming the stent may comprise a first proximal segment having proximal and distal ends and a first outer diameter, a second distal segment having proximal and distal ends and a second outer diameter smaller than the said first outer diameter, and a third intermediate segment having a proximal end connected to the distal end of the first segment and a distal end connected to the proximal end of the second segment. With such a configuration the stent has a differential geometry which allows a-very strong anchor of the first proximal segment in the body passageway due to the higher radial force at that level. The third intermediate segment gives to the braiding a varying steep angle with respect to the longitudinal axis of the tubular wall which raises flexibility and/or radial force depending on the relative size of stent and vessel and on the elasticity of vessel wall; this structure also strongly limits any flattening deformation tendency whereby the deformation of the stent section remains closer to a circle. The second distal segment makes an easier and safer way through curves or at the end of a pipe. The differential geometry thus allows a higher flexibility where needed, i.e., before a curve of the body passageway, and it provides a better bend taking, a smoother way in the curve, and a better force differential to avoid migration under movements of the vessel or when the stent is placed in delicate locations such as the junction of the esophagus with the stomach.
Where the first proximal and second distal segments are cylindrical, the first proximal segment may firmly anchor in the vessel without any risk of damage to the vessel wall or to possible fistulas because of the surface repartition of the pressure of the braiding against the vessel wall, whereas the second distal segment may smoothly bear against the vessel wall, even in strongly narrowed areas.
Where the third intermediate segment is a truncated cone having a base forming the proximal end of the third intermediate segment and a top forming the distal end of the third intermediate segment, the best transitional flexibility and/or radial force repartiton is achieved between the first proximal and second distal segments. And when the third intermediate segment is formed of a plurality of consecutive truncated cones connected to one another with each of said truncated cones having a taper oriented towards the distal end of the intermediate segment, with the possibility of having two or more consecutive cones separated by a cylindrical segment connected thereto, stents may be manufactured to meet specific requirements of flexibility, radial force, shaping up and selective anchor in particular conditions of body vessels.
A covering layer of elastic material may surround the tubular wall to prevent ingrowth of unwanted cells through the stent. In a preferred embodiment, a covering layer of elastic material is arranged within the tubular wall to also prevent ingrowth of unwanted cells through the stent; and the stent also enjoys a stronger anchor of its segments in the body cavity due to the direct contact of the braiding therewith. Within this frame, a distal portion of the second distal segment may be uncovered by the covering layer to assure when required a better gripping of the stent to the body cavity in that area, because of the stronger interpenetration between braiding and vessel wall. In a still preferred embodiment, at least a proximal portion of the first proximal segment is not covered by the covering layer to enhance by stronger interpenetration between braiding and vessel wall the essential gripping of the stent in the body passageway in that area of higher radial force. Such an uncovering of the first proximal segment may extend the full length of the first proximal segment to take full advantage of the higher radial force to ensure the safest anchor of that segment in the body passageway. The uncovering of the first proximal segment also prevents food trapping at the ingress of the stent between the first proximal segment and the vessel wall; it also allows a better fluid ingress through the stent if the first proximal segment is somewhat bent in the vessel and does not completely apply there against. And to provide a further safety anchor of the stent in the body passageway, the proximal end of the first proximal segment and/or the distal end of the second distal segment may be flared up.
According to a first method for manufacturing the stent, it is provided to form an elongated mandrel having a first proximal segment having proximal and distal ends and a first outer diameter, a second distal segment having proximal and distal ends and a second outer diameter smaller than said first outer diameter, and a third intermediate segment having a proximal end connected to the distal end of the first proximal segment and a distal end connected to the proximal end of the second segment, to form an elongated tubular braid of spring steel having proximal and distal ends and an inner diameter greater than said first outer diameter of the first segment of the mandrel, to engage said tubular braid over the mandrel, to heat the tubular braid over the mandrel, and to pull during the heating the proximal and distal ends of the tubular braid away from one another on the mandrel to closely radially contract the tubular braid over the segments of the mandrel. As the spring steel of the tubular braid needs anyhow a heat treatment to properly perform the resiliency of the braid, this method takes advantage of this compulsory treatment and of the deformation capacity of the braid to form the differential geometry of the stent in a simple, economical and efficient manner.
According to a second method for manufacturing the stent, it is provided to form an elongated tubular mandrel having a first proximal hollow segment having proximal and distal ends and a first inner diameter, a second distal hollow segment having proximal and distal ends and a second inner diameter smaller than said first inner diameter, and a third intermediate hollow segment having a proximal end connected to the distal end of the first hollow segment and a distal end connected to the proximal end of the second hollow segment, to form an elongated tubular braid of spring steel having proximal and distal ends and an outer diameter greater than the first inner diameter of the first hollow segment of the tubular mandrel, to pull the proximal and distal ends of the elongated tubular braid away from one another to radially contract the tubular braid, to engage the contracted tubular braid into the tubular mandrel, to release the pull on the ends of the tubular braid to radially expand it in the mandrel, and to heat the tubular braid in the mandrel to closely radially expand the tubular braid against the segments of the tubular mandrel. As for the first method, this method basically takes advantage of the need of a heat treatment for the braid to properly perform its resiliency to form the differential geometry of the stent; and in addition the method takes advantage of the self expansion capacity of the braid to form the stent in a simple, economical and efficient manner.
In sum, the present invention relates to a stent for use in a body passageway. A flexible self-expandable braided tubular wall has a first proximal segment having proximal and distal ends and a first outer diameter, a second distal segment having proximal and distal ends and a second outer diameter smaller than said first outer diameter, and a third intermediate segment having a proximal end connected to the distal end of the first segment and a distal end connected to the proximal end of the second segment. The first proximal and second distal segments may be cylindrical. The third intermediate segment may be a truncated cone having a base forming the proximal end of the third intermediate segment and a top forming the distal end of the third intermediate segment. The third intermediate segment may be formed of a plurality of consecutive truncated cones connected to one another, each of said truncated cones having a taper oriented towards the distal end of the intermediate segment. At least two of said consecutive cones may be separated by a cylindrical segment connected thereto. The stent may also have a covering layer of elastic material surrounding the tubular wall, which may be arranged within said tubular wall. At least a proximal portion of the first proximal segment may not be covered by the covering layer. A distal portion of the second distal segment may not be covered by the covering layer. The proximal end of the first proximal segment and/or the distal end of the second distal segment may be flared up.
The present invention also relates to a method for manufacturing a stent, including the steps of: (1) forming an elongated mandrel having a first proximal segment having proximal and distal ends and a first outer diameter, a second distal segment having proximal and distal ends and a second outer diameter smaller than the first outer diameter, and a third intermediate segment having a proximal end connected to the distal end of the first segment and a distal end connected-to the proximal end of the second segment; (2) forming an elongated tubular braid of spring steel having proximal and distal ends and an inner diameter greater than the first outer diameter of the first segment of the mandrel; (3) engaging the tubular braid over the mandrel; (4) heating the tubular braid over the mandrel; and, (5) pulling during said heating the proximal and distal ends of the tubular braid away from one another on the mandrel to closely radially contract the tubular braid over the segments of the mandrel.
The present invention also relates to a method for manufacturing the stent, including the steps of (1) forming an elongated tubular mandrel having a first proximal hollow segment having proximal and distal ends and a first inner diameter, a second distal hollow segment having proximal and distal ends and a second inner diameter smaller than the first inner diameter, and a third intermediate hollow segment having a proximal end connected to the distal end of the first hollow segment and a distal end connected to the proximal end of the second hollow segment; (2) forming an elongated tubular braid of spring steel having proximal and distal ends and an outer diameter greater than the first inner diameter of the first hollow segment of the tubular mandrel; (3) pulling the proximal and distal ends of the elongated tubular braid away from one another to radially contract the tubular braid; (4) engaging the contracted tubular braid into the tubular mandrel; (5) releasing the pull on the ends of the tubular braid to radially expand it in the mandrel; and, (6) heating the tubular braid in the mandrel to closely radially expand the tubular braid against the segments of the tubular mandrel.